The light weight and strength of light metals and their alloys and especially of magnesium and magnesium alloys makes products fashioned therefore highly desirable for use in manufacturing critical components of, for example, aircrafts, terrestrial vehicles and electronic devices. One of the most significant disadvantages of magnesium and magnesium alloys is corrosion. Exposure to corrosive or oxidizing conditions causes magnesium and magnesium alloy surfaces to corrode rather quickly, corrosion that is both unaesthetic and reduces strength.
There are many methods for improving the corrosion resistance of a magnesium and magnesium alloy workpiece by modifying the surface of the workpiece. It is generally accepted that the best corrosion resistance for magnesium and magnesium alloy surfaces is achieved by anodizing. In anodizing, a metallic workpiece is used as an anode of an electrical circuit. The circuit includes an electrolyte bath in which the workpiece is contacted, mostly by immersing, seldom by spraying. Depending on the properties of the current used, the bath temperature and the composition of the electrolyte bath, the surface of the workpiece is modified in various ways.
Various aqueous solutions and various additives had been found in, for example: U.S. Pat. No. 4,023,986 (trihalogenated compounds and a group 1b, 2, 3a, 4b, 5b, 6b and 8 metal and an arylamine); U.S. Pat. No. 4,184,926 (alkali metal silicate and alkali metal hydroxide solution); U.S. Pat. No. 4,551,211 (aluminate and alkali hydroxide and boron/sulfate/phenol/iodine solution); U.S. Pat. No. 4,620,904 (basic silicate and hydroxide and fluoride solution); U.S. Pat. No. 4,978,432 (alkaline pH with borate/sulfonate, phosphate and fluoride/chloride solution); U.S. Pat. No. 5,264,113 (alkaline pH with fluoride containing aqueous solution followed by alkaline solution with hydroxide, fluoride and silicate); U.S. Pat. No. 5,470,664 (neutral NH4F solution followed by alkaline solution containing hydroxide, fluoride/fluorosilicate and silicate); U.S. Pat. No. 5,792,335 (ammonia and phosphate containing aqueous solution with an optional content of ammonium salts and of peroxides); and U.S. Pat. No. 6,280,598 (aqueous solution with various amines/ammonia and phosphate/fluoride with optional sealing agents).
Although anodizing is effective in increasing the corrosion resistance and the hardness of the surface, the anodizing coating does not up to now fulfill all requirements expected.
The metallic surfaces coated with an anodizing coating usually become very rough. The anodizing coatings show typically many pores caused by sparking during the anodizing procedure, especially in combination with break-downs or bigger flames. These pores trap humidity and other corrosion-inducing agents. Upon exposure to extreme conditions, humidity is trapped in the pores leading to corrosion. The use of ammonia or amine in the solutions as taught in U.S. Pat. No. 5,792,335 and in U.S. Pat. No. 6,280,598 apparently prevents sparking, leading to smaller pores. However, the coatings built in so called “non-spark processes” only have a low thickness, which is often in the range from about 3 to about 5 μm and have often a low wear resistance. The use of a high concentration of ammonia in an anodizing solution makes it almost impossible to apply this solution in industry without expensive equipment as there is a strong poisonous smell so that there has to be an equipment of closed chambers with exhaustion. In U.S. Pat. No. 6,280,598, it is explicitly stated that the use of alkali hydroxide salts is not preferred in an anodizing solution. There, the occurrence of sparking during the anodizing is discouraged because of several undesirable phenomena mentioned in columns 1 and 2.
It would be highly advantageous to have a method for treating metallic surfaces which are anodizable like surfaces of magnesium, magnesium alloys, aluminum, aluminum alloys, titanium, titanium alloys, beryllium or of beryllium alloys so as to have a high corrosion and wear resistance. It would be favorable if then anodizing coatings would be generated with a low roughness, with a reduced number of big pores or with smaller pores. Further on, it is preferable that such a treatment is environmentally friendly and does not include—as far as possible—fluorides, ammonia, heavy metals and other hazardous components.